1. Field of the Invention
This invention relates to a rubber adherable steel cord adapted for reinforcement of resilient articles, such as rubber hoses, rubber belts or vehicle tyres.
2. Related Art
For these applications, such cord will generally be a structure of steel wires, twisted appropriately, the wires having a diameter ranging from 0.03 to 0.80 mm, in general in the range from 0.14 to 0.40 mm, and the steel having a tensile strength of at least 2000N/mm.sup.2 and an elongation at rupture of at least 1%, preferably about 2%, being in general carbon steel (preferably 0.65 to 0.95% carbon) in its ferritic state. For these applications, the cord will generally further comprise, in order to obtain the necessary rubber adherability for reinforcement purposes, a rubber-adherable coating, such as copper, zinc, brass or ternary brass alloy, or a combination thereof, the coating having a thickness ranging from 0.05 to 0.40 micron, preferably from 0.12 to 0.22 micron. The coating can also be present in the form of a thin film of chemical primer material for ensuring good rubber penetration and adhesion.
The wires are twisted into a bundle according to a given structure, e.g. twisted strands or superposed layers, and this bundle may or may not be provided with a wrapping filament, helicoidally wound around the bundle. In determining below any twisting structure and number of filaments, this wrapping filament is not taken into consideration, and may or may not be present in addition.
For truck tyre belt and carcass in particular, the requirements for a suitable cord structure are specifically: high tensile strength (which needs a structure with a minimum of cabling loss), good compactness (in order to obtain thin reinforcement plies, necessary specifically in the belt area of the tyre), high fatigue resistance (by inter alia less fretting in the contact points between wires), low moisture penetration possibility (for corrosion resistance), and simple manufacturing method (for reduced costs). For this use, the cords generally have a steel cross-sectional area ranging from 0.5 to 3.5 mm.sup.2.
For meeting these requirements, a cord has been developed of the 7.times.4.times.0.22 SZ-type, which means: a structure of 7 strands twisted around each other in the S-direction, each strand comprising 4 wires of 0.22 mm diameter twisted around each other in the Z-direction. But cord manufacturers are in continuous search for improved cord structures, trying to reconcile in a still better way the often contradictory requirements for such cord.
In this respect, a 3+9+15.times.0.22 SSZ-cord is known (which means: a core of three wires twisted around each other in the S-direction, surrounded by a layer of nine wires, twisted around the core in the S-direction, the whole being surrounded by another layer of fifteen wires, twisted in the Z-direction, all wires having a diameter of 0.22 mm) developed as an alternative for the 7.times.4 type, in particular for its lower cabling loss, better compactness and less fretting.
In the search to even better structures, a further alternative has been proposed, consisting of a single-bundle 27.times.1 structure in a compact configuration and with a longitudinally regular twist. By a 27.times.1-structure is meant a bundle of 27 wires, all twisted in the same direction and with the same pitch. By a "compact" configuration is meant that the transverse section of the cord shows a number of nearly circular wire cross-sections of the same diameter (neglecting the fact that the wires are not perfectly perpendicular to the cord cross-section, which leads to a slightly elliptic form), arranged in a close-packed array so that, when the centres of all these circles are connected, there is formed a network of equilateral triangles of which the sides are equal to the wire diameter. By a "longitudinally regular twist" is meant that in the longitudinal direction, successive transverse sections show the same or similar configurations, although phase-shifted with respect to each other, i.e. the cross-section of each wire is in the same position in the array with respect to the cross-section of the other wires, although there will be a shift of the whole configuration, i.e. a rotation around the centres of the transverse sections, due to the twist, through an angle which is proportional to the distance between successive transverse sections. The configurations in all transverse sections are thus in principle identical, but due to some inevitable twisting imperfections, in practice any wire cross-section can be shifted from its ideal position (where it should be when the configurations would be identical, by a distance of about one fourth of a cord diameter) in which case the configurations are called "similar".
In the 27.times.1 compact cord with longitudinally regular twist above, one can distinguish: a central bundle of steel wires and a circumferential layer of steel wires, (a "layer" having only a thickness of one wire diameter) helicoidally twisted around said central bundle, the latter showing in transverse cross-section a core of adjacent wire cross-sections, surrounded by a ring of wire cross-sections (a "ring" being meant to be a succession one after another along a generally circular path, so that the ring can only have a width of one wire diameter).
In this compact and regular structure, which can be made simply in a single twisting operation, adjacent helicoidal wires are stacked together in their most compact configuration in perfect parallellism, contacting each other along a line instead of in cross-points, so that fretting is very low. Such compactness also results in a better resistance to cutting as reflected by an impact test. Unfortunately however, this cord produces the phenomenon of "wire migration". The cords are generally used in practice in e.g. tyre plies in the form of cut lengths of 35-55 cm, and in running tests of a tyre, one or more wires have been found to shift lengthwise with respect to their neighbours, and emerge at one end of the cord, at one side of the ply, over a certain length, puncturing through the rubber and damaging the tyre. For this reason, this latter cord does not seem to be a good candidate to replace the 7.times.4 or the 3+9+15 cord mentioned above.